A variety of methods has heretofore been proposed as to the heat-sensitive recording system which provides a record of images or marks by utilizing a physical or chemical change that a substance undergoes under the influence of thermal energy. Among these heat-sensitive recording methods, the so-called chromogenic reactant based color production-type heat-sensitive recording method and heat-sensitive recording paper making use of the recording method, which are proposed in Japanese Patent Publicattion No. 14039/1970, etc., have found wide-spread commercial utility in output console units such as computer console units and facsimiles, reflecting recent technical advancement in thermal printers led by the development of thermal heads making use of resistor elements, because the above particular heat-sensitive recording method is free of such problems as noise and offensive odor and it permits high-speed recording. Accordingly, the chromogenic reactant based color production-type heat-sensitive recording method and heat-sensitive recording paper which makes use of the above recording method are expected to find still more acceptance in the future.
These chromogenic reactant based color production-type heat-sensitive recording paper are each formed, generally speaking, of (A) a lactone-type chromogenic reactant, for example, 3-bis(4-dimethylaminophenyl)-6-dimethylaminophthalide (Crystal Violet Lactone) and (B) an acidic substance, for example, a phenolic compound such as 2-bis(4-hydroxyphenyl)propane (bisphenol A), which are supported together with a binder and other additives on a base. An application of heat causes at least one of the reactants (A) and (B) to melt and the reactants (A) and (B) are hence allowed to contact with each other, thereby forming a color image or mark with the thus-acidified lactone-layer chromogenic reactant on the base.
Heat-sensitive recording paper making use of the color production by the contact of a lactone-type chromogenic reactant and an acidic phenolic compound is however accompanied by the following serious problems:
(1) It is necessary to carry a chromogenic reactant and its matching color-developing agent both in large amounts on a base if one wants to obtain a color image or mark having sufficient density. A lot of energy is thus required to melt either one or both of the chromogenic reactant and color-developing agent so as to bring them into mutual contact, thereby making the heat-sensitive recording paper difficult to meet the recent trend toward still faster recording in data communication (facsimiles); and
(2) Color images or marks, which are obtained by the reaction between an chromogenic reactant and color-developing agent when either one or both of the chromogenic reactant and color-developing agent are melted by heat, are extremely unstable and are susceptible of undergoing easy discoloration or fading upon exposure to light, whereby making the color images or markes illegible. Furthermore, such color images or marks readily disappear upon contact with an oil or the like.
There is a strong outstanding demand for the improvement to these problems in the above heat-sensitive recording method which is expected to become the dominant method in facsimiles which have been finding more and more acceptance in recent years. A variety of extensive researches and investigations have been made in recent years with respect to chromogenic reactants, color-developing agents, additives and coating formulations for heat-sensitive recording paper. Under the circumstances, none of such researches and investigations appear to have resulted in any breakthrough improvement.